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Best Way to build a calibrated S meter
I was wondering how commercial equipment go about calibrating the S
meter and whats the best way of building a calibrated S meter that would be reliable as a professional field strength meter? Since most S meters just read AGC voltage and it would be easy to build some interface with a CPU to read voltages and calibrate this way. The question is this, is a S meter calibrated this way actually reading in a accurate way whats occurring at the antennas terminal. Since most antenna inputs are not 50 ohms J0, how would you design a system like a spectrum analyzer that measures signal voltage at the antenna terminal?. I would be interested in some ideas, i am homebrewing a shortwave receiver however i wanted a calibrated S meter in Dbuv, Dbm, S units and Millivolts. I also want to use a calibrated antenna for a Antenna factor input to have a meaningful long term view of propagation signal strength. Thanks Will |
Best Way to build a calibrated S meter
Replace the meter scale with a white card and calibrate it yourself
with pen and ink. You will also need a signal generator and an attenuator. Don't forget, an S-meter is a power or wattmeter. It indicates signal STRENGTH. S9 = 50 pico-watts. ---- Reg. |
Best Way to build a calibrated S meter
wrote Only by coincidence and only in a 50 ohm system elsewhere S9 is 50 uV (microVolts) equals 50pW. ========================================== Allison, I'm afraid you are not quite correct about coincidences. The beauty of calibrating an S-meter in watts is that the result is independent of receiver impedance. Of course, there must be a conjugate match between antenna and receiver. And there usually is. This is taken care of in the calibration process. The received signal STRENGTH is indicated in watts, which is all one wants to know. When reading the meter, who cares about what impedance the measuring instrument happens to be? To sum up : There is a transmitter of given power output. There is a radio path which is an attenuator, And there is a received signal signal strength meter which indicates watts. The overall loss between transmitter and power meter may be deduced in terms of decibels. Professional radio engineers do it all the time. It can be misleading to think in terms of S9 = 50 uV when one doesn't know what the receiver input impedance is. ---- Reg, G4FGQ |
Best Way to build a calibrated S meter
Reg Edwards wrote:
Replace the meter scale with a white card and calibrate it yourself with pen and ink. You will also need a signal generator and an attenuator. Don't forget, an S-meter is a power or wattmeter. It indicates signal STRENGTH. S9 = 50 pico-watts. ---- Reg. Yeah thats for that tip. But the question is how do you actually build a piece of electronics that measures signal levels at the receiver terminals. Or is it reading agc voltage just as accurate. The problem the method you described is that it will vary from band to band. I just wonder how field strength meters are designed, especially the new solid state ones. Spectrum analyzers too have a pretty flat response. I want to build this ability into my homebrew receiver much the same way a selective level meter works. Maybe someone has done it. Will |
Best Way to build a calibrated S meter
On Tue, 23 May 2006 12:16:27 +0100, "Reg Edwards"
wrote: Replace the meter scale with a white card and calibrate it yourself with pen and ink. You will also need a signal generator and an attenuator. If you derive the S-meter reading from the a.g.c. control voltage, be prepared to perform the calibration at various temperatures, since the gain of simple IF-strips vary with temperature (and thus need a different amount of a.g.c.), unless the gain is controlled by passive components only. With various front end filter for various frequency bands and attenuation depending of the frequency, you would have to perform the calibration on several frequencies. Instead of a pen and ink, it would be more practical to use a micro controller with sufficient non-volatile storage for the conversion tables and use analog inputs for a.g.c. voltage and temperature and some kind of input for the frequency being received. With these inputs and the conversion table, a voltage proportional of the log of the antenna terminal power could be generated. Paul OH3LWR |
Best Way to build a calibrated S meter
"Will" wrote I just wonder how field strength meters are designed, especially the new solid state ones. Spectrum analyzers too have a pretty flat response. I want to build this ability into my homebrew receiver much the same way a selective level meter works. Maybe someone has done it. =========================================== The accuracy of S-meters varies from one band to another because the receiver input impedance varies from one band to another. Even from one end of a band to the other, especially if the input stage is tuned. Also, the stage-gain of an RF stage can vary very much from one band to another. To ensure a consistent gain it is necessary to convert to the intermediate frequency (IF) at the very first stage of a receiver. Preferably using a high-level, balanced diode-bridge, modulator. The IF stages can be assumed to have a constant, wide-band gain, except, of course, for the automatic gain control (AGC) action. Then ensure that the receiver input impedance is constant (usually 50 ohms) over the whole HF range of the receiver. Remember the S-meter is a power or watt-meter. It so happens that with dual-gate FET's, and 3 or 4 IF amplifier stages, a meter which responds to AGC volts will fairly accurately indicate S-units and dB above S9 in a linear fashion. However, below about S4 the linearity of S-meter readings begins to fail. And you will have to make and calibrate your own meter scale. There's no way of avoiding it! That is unless you can mentally visualise what the scale ought to look like as you use it. Accurate commercial field strength meters use antennas dedicated to the job, conjugate-matched to the receiver, are insensitive to weak signals, are narrow band and are very expensive. To obtain an S-meter scale to be proud of, you will have to use pen and ink with a signal generator and 100-dB stepped attenuator. Attempts to calibrate the scale with clever, highly complicated electronics will get you nowhere in a long time. And will cost you more than the remainder of the receiver. Sorry to be so despondent. ---- Reg, G4FGQ |
Best Way to build a calibrated S meter
"Paul Keinanen" wrote in message ... On Tue, 23 May 2006 12:16:27 +0100, "Reg Edwards" wrote: Replace the meter scale with a white card and calibrate it yourself with pen and ink. You will also need a signal generator and an attenuator. If you derive the S-meter reading from the a.g.c. control voltage, be prepared to perform the calibration at various temperatures, since the gain of simple IF-strips vary with temperature (and thus need a different amount of a.g.c.), unless the gain is controlled by passive components only. With various front end filter for various frequency bands and attenuation depending of the frequency, you would have to perform the calibration on several frequencies. Instead of a pen and ink, it would be more practical to use a micro controller with sufficient non-volatile storage for the conversion tables and use analog inputs for a.g.c. voltage and temperature and some kind of input for the frequency being received. With these inputs and the conversion table, a voltage proportional of the log of the antenna terminal power could be generated. Paul OH3LWR =========================================== I don't doubt that the problem is capable of being solved. Just throw enough electronic hardware and software at it. The cost can be estimated at 100,000 Euros. The size and weight of the receiver will be quadrupled. No amateur would be able to afford buying one. American tax-payers would prevent NASA from doing the research and development work. Even Bush would object. As for me and most other amateurs, I'm quite happy with an S-meter reading which puts signal strength reports in the right ball-park. Very strong, very weak, or indifferent. ---- Reg. |
Best Way to build a calibrated S meter
Reg Edwards wrote:
"Will" wrote I just wonder how field strength meters are designed, especially the new solid state ones. Spectrum analyzers too have a pretty flat response. I want to build this ability into my homebrew receiver much the same way a selective level meter works. Maybe someone has done it. =========================================== The accuracy of S-meters varies from one band to another because the receiver input impedance varies from one band to another. Even from one end of a band to the other, especially if the input stage is tuned. Also, the stage-gain of an RF stage can vary very much from one band to another. To ensure a consistent gain it is necessary to convert to the intermediate frequency (IF) at the very first stage of a receiver. Preferably using a high-level, balanced diode-bridge, modulator. The IF stages can be assumed to have a constant, wide-band gain, except, of course, for the automatic gain control (AGC) action. Then ensure that the receiver input impedance is constant (usually 50 ohms) over the whole HF range of the receiver. Remember the S-meter is a power or watt-meter. It so happens that with dual-gate FET's, and 3 or 4 IF amplifier stages, a meter which responds to AGC volts will fairly accurately indicate S-units and dB above S9 in a linear fashion. However, below about S4 the linearity of S-meter readings begins to fail. And you will have to make and calibrate your own meter scale. There's no way of avoiding it! That is unless you can mentally visualise what the scale ought to look like as you use it. Accurate commercial field strength meters use antennas dedicated to the job, conjugate-matched to the receiver, are insensitive to weak signals, are narrow band and are very expensive. To obtain an S-meter scale to be proud of, you will have to use pen and ink with a signal generator and 100-dB stepped attenuator. Attempts to calibrate the scale with clever, highly complicated electronics will get you nowhere in a long time. And will cost you more than the remainder of the receiver. Sorry to be so despondent. ---- Reg, G4FGQ Thanks Reg for the summary. I think the Flex radio and the Winradio have the ability to be calibrated accurately via a look up table. I always admired shortwave receivers like the R&S EK 2000 which had a accurate meter calibrated in the DbUv scale. I think i will just have to abandon my super accurate S meter project. Will |
Best Way to build a calibrated S meter
Will take a look at URL:
http://www.ac6v.com/sunit.htm Maybe the National Semiconductor NE604 IF amplifier IC would be worth looking into. Claims say it provides an accurate signal strength logarithmic output that closely tracks the input signal level over a wide dynamic range that could possibly be used for driving an S meter circuit. -- CL -- I doubt, therefore I might be ! "Will" wrote in message ... I was wondering how commercial equipment go about calibrating the S meter and whats the best way of building a calibrated S meter that would be reliable as a professional field strength meter? Since most S meters just read AGC voltage and it would be easy to build some interface with a CPU to read voltages and calibrate this way. The question is this, is a S meter calibrated this way actually reading in a accurate way whats occurring at the antennas terminal. Since most antenna inputs are not 50 ohms J0, how would you design a system like a spectrum analyzer that measures signal voltage at the antenna terminal?. I would be interested in some ideas, i am homebrewing a shortwave receiver however i wanted a calibrated S meter in Dbuv, Dbm, S units and Millivolts. I also want to use a calibrated antenna for a Antenna factor input to have a meaningful long term view of propagation signal strength. Thanks Will |
Best Way to build a calibrated S meter
|
Best Way to build a calibrated S meter
That's why for the HF bands S9 is defined as a receiver input signal of
50 microvolts at 50 Ohms input impedance , translated as -73dBm For frequencies higher than 30 MHz S9 is defined as 5 microvolts at 50 Ohms input impedance , translated as -93dBm. The S9 signal strength was first set by Art Collins of Collins Radio fame during the 1940s and was later accepted by IARU. This organisation also adopted the 5 microvolt level for a S9 signal for VHF and higher frequency bands. Frank GM0CSZ / KN6WH ================================ It can be misleading to think in terms of S9 = 50 uV when one doesn't know what the receiver input impedance is. ---- Reg, G4FGQ |
Best Way to build a calibrated S meter
To obtain an S-meter scale to be proud of, you will have to use pen and ink with a signal generator and 100-dB stepped attenuator. Attempts to calibrate the scale with clever, highly complicated electronics will get you nowhere in a long time. And will cost you more than the remainder of the receiver. Sorry to be so despondent. ---- Reg, G4FGQ ================================ Just an idea ; if one is really 'dead keen' to have an 'accurate ? ' S-meter readout, you could construct an indicator with individual threshold opamp comparators ( up to 4 in a DIL package) each driving a LED. When calibrating with a signal gen as a 50 Ohms source ,starting with S-9 being 50 microvolts being -73dBm (or 5 microvolts being -93dBm for VHF and higher) the relevant LEDs can then be set separately below that level with 6dB steps and above S-9 with 10 dB steps by accepting the quasi-log voltage range generated by the AGC as fed to the traditional analogue S-meter A fancy feature would be different colour LEDs showing signal strength above S-9. I feel that ,while accepting any 'professional' comments,this would be a practical 'amateur ' (low cost) solution . Frank GM0CSZ / KN6WH an 'Amateur' |
Best Way to build a calibrated S meter
"Caveat Lector" wrote in message news:64Gcg.177511$bm6.76448@fed1read04... Will take a look at URL: http://www.ac6v.com/sunit.htm Maybe the National Semiconductor NE604 IF amplifier IC would be worth looking into. Claims say it provides an accurate signal strength logarithmic output that closely tracks the input signal level over a wide dynamic range that could possibly be used for driving an S meter circuit. -- CL -- I doubt, therefore I might be ! This is ok, but the one thing is that any Signal strength IC like the 604 has a constant slope and the S-Meter will be rather compressed in the 0-9 s-units range compared to the over S9 range (obviously fixable with meter scale). I don't recall the 604's dynamic range. You could make a two slope correction above S9, but I'd have to do some scratch paper work to figure out the circuit [ probably as simple as a resister and diode in the right place]. Or use two of them and change the gain into one to get the different slope. I don't recall the 604's dynamic range. I have some SLx16...oops can't remember the numbers (? 1316?... 1613? ) , that are cascaded for large dynamic range log amps. Just some ideas. 73, Steve, K9DCI |
Best Way to build a calibrated S meter
The way that spectrum analyzers are built is typically to use
calibrated attenuators and linear gain stages that have very low distortion (and similarly low "compression"). Then it becomes a matter of determining the voltage after amplification. They do NOT use AGC voltage, or at least not anything like the AGC used in a typical ham receiver. The way we do it here is to digitize the RF signal and do some appropriate digital signal processing on it (e.g., FFT) to display the spectrum and to calculate amplitudes and band powers and the like. Modern digitizers are very linear indeed and can be used to measure signal amplitudes over a range in excess of 120dB with relative accuracy far better than an S meter over most of that range, and still considerably better even at the bottom end of the range. It doesn't even take a huge number of bits in the digitization to do it; consider that a typical delta-sigma ADC is a one-bit converter followed by lots of processing gain. The way it can be done "on the cheap" is to use a calibrated attenuator and a single known signal level. Then you compare your known signal level with the unknown, adjusting the attenuator to bring your (typically large) signal down to the same amplitude as the unknown. For S-meter levels of accuracy, linear non-AGC'd stages feeding one of the RF power detectors from Analog Devices, Linear Technology or others will work fine. Most of them have an output voltage proportional to the log of the input voltage, and so can be calibrated to read dB linearly on a linear meter scale. If your receiver has a good front end, it shouldn't need AGC up through the filter following the mixer, and you could pick off there after the filter to drive the meter circuit. That seems overkill, but it would get you a _good_ S-meter. Then you'd have to calibrate out the front-end gain at least per band, assuming you have at least some front end filtering that doesn't have the same gain (loss) on each band. Field strength meters that accurately measure an RF electromagnetic field are basically spectrum analyzers fed by calibrated antennas. That may be beyond what you wanted to know or do, but it should give you a pretty accurate picture of how modern commercial gear actually does make RF voltage measurements. You could add calibration (for absolute amplitude accuracy as well as spectral flatness) to all that as a whole 'nuther topic, though. For example, the amplitude characteristics of any filters the signal passes through in the spectrum analyzer must be properly accounted for, as must temperature drifts in instruments with high accuracy. Cheers, Tom |
Best Way to build a calibrated S meter
I think I'd follow K7ITM's advice.
I'd split the signal right after the IF filters. Feed one of them into the rest of the Rx chain, and the other one into one of the Analog devices logarithmic detector chips. This is assuming there is no AGC prior to the IF filters. There are two major choices for the detector chip, AD8307, which is a power detector, or a true RMS detector (AD8326??) Feed the output into a A/D coinverter, and then digitally add correction factors for the front end gain and possible different sensitivity on different bands. So, once you have the calibration factors, this wouls be a fairly accurate iindicator of the input signal. Granted, it is assuming a matched 50 ohm antenna, and you will get an error due to antenna reactances and so on. However, it is as precise as an amateur radio system is likely to be able to get and a heck of a lot more meaningful than most 'S' meters I've seen. Jim N6BIU |
Best Way to build a calibrated S meter
K7ITM wrote:
The way that spectrum analyzers are built is typically to use calibrated attenuators and linear gain stages that have very low distortion (and similarly low "compression"). Then it becomes a matter of determining the voltage after amplification. They do NOT use AGC voltage, or at least not anything like the AGC used in a typical ham receiver. The way we do it here is to digitize the RF signal and do some appropriate digital signal processing on it (e.g., FFT) to display the spectrum and to calculate amplitudes and band powers and the like. Modern digitizers are very linear indeed and can be used to measure signal amplitudes over a range in excess of 120dB with relative accuracy far better than an S meter over most of that range, and still considerably better even at the bottom end of the range. It doesn't even take a huge number of bits in the digitization to do it; consider that a typical delta-sigma ADC is a one-bit converter followed by lots of processing gain. The way it can be done "on the cheap" is to use a calibrated attenuator and a single known signal level. Then you compare your known signal level with the unknown, adjusting the attenuator to bring your (typically large) signal down to the same amplitude as the unknown. For S-meter levels of accuracy, linear non-AGC'd stages feeding one of the RF power detectors from Analog Devices, Linear Technology or others will work fine. Most of them have an output voltage proportional to the log of the input voltage, and so can be calibrated to read dB linearly on a linear meter scale. If your receiver has a good front end, it shouldn't need AGC up through the filter following the mixer, and you could pick off there after the filter to drive the meter circuit. That seems overkill, but it would get you a _good_ S-meter. Then you'd have to calibrate out the front-end gain at least per band, assuming you have at least some front end filtering that doesn't have the same gain (loss) on each band. Field strength meters that accurately measure an RF electromagnetic field are basically spectrum analyzers fed by calibrated antennas. That may be beyond what you wanted to know or do, but it should give you a pretty accurate picture of how modern commercial gear actually does make RF voltage measurements. You could add calibration (for absolute amplitude accuracy as well as spectral flatness) to all that as a whole 'nuther topic, though. For example, the amplitude characteristics of any filters the signal passes through in the spectrum analyzer must be properly accounted for, as must temperature drifts in instruments with high accuracy. Cheers, Tom Thanks Tom and Jim. The information you provided has given me something to think about. Even though the task is complex it can be done. I will experiment with some ideas and see if i can find a sollution. Regardless how hard it is i think its worthwile pursuing a accurate S meter. Will |
Best Way to build a calibrated S meter
On Wed, 24 May 2006 15:55:36 +1000, Will wrote:
Regardless how hard it is i think its worthwile pursuing a accurate S meter. Will It will make you very unpopular. People who are used to getting S9+30dB reports get quite upset when you give the a 57 :-) 73, Ed. EI9GQ. -- Linux 2.6.16 Remove 'X' to reply by e-mail. Yes, my username really is: nospam |
Best Way to build a calibrated S meter
Regardless how hard it is i think its worthwile pursuing a accurate S
meter. Will It will make you very unpopular. People who are used to getting S9+30dB reports get quite upset when you give the a 57 :-) ====================================== Why would a serious radio amateur trying hard to use his equipment in the best way possible be required to be popular ? Frank GM0CSZ / KN6WH |
Best Way to build a calibrated S meter
Highland Ham wrote:
It will make you very unpopular. People who are used to getting S9+30dB reports get quite upset when you give the a 57 :-) ====================================== Why would a serious radio amateur trying hard to use his equipment in the best way possible be required to be popular ? Frank GM0CSZ / KN6WH Zoom! |
Best Way to build a calibrated S meter
On Tue, 23 May 2006 16:26:43 +0100, "Reg Edwards"
wrote: "Paul Keinanen" wrote in message .. . On Tue, 23 May 2006 12:16:27 +0100, "Reg Edwards" wrote: Replace the meter scale with a white card and calibrate it yourself with pen and ink. You will also need a signal generator and an attenuator. If you derive the S-meter reading from the a.g.c. control voltage, be prepared to perform the calibration at various temperatures, since the gain of simple IF-strips vary with temperature (and thus need a different amount of a.g.c.), unless the gain is controlled by passive components only. With various front end filter for various frequency bands and attenuation depending of the frequency, you would have to perform the calibration on several frequencies. Instead of a pen and ink, it would be more practical to use a micro controller with sufficient non-volatile storage for the conversion tables and use analog inputs for a.g.c. voltage and temperature and some kind of input for the frequency being received. With these inputs and the conversion table, a voltage proportional of the log of the antenna terminal power could be generated. Paul OH3LWR =========================================== I don't doubt that the problem is capable of being solved. Just throw enough electronic hardware and software at it. The cost can be estimated at 100,000 Euros. The size and weight of the receiver will be quadrupled. No amateur would be able to afford buying one. American tax-payers would prevent NASA from doing the research and development work. Even Bush would object. While NASA might have needed such sums in the 1960's, the functionality suggested above should be doable these days with any micro controller with a few kilobytes of non-volatile storage. While the frequency, power level and temperature calibration points might require a three dimensional array, in practice, it should be acceptable to get a few single dimensional vectors for each amateur band. One vector for a specific band might contain the front end frequency response for 5-10 frequencies within and around a band. Getting the a.g.c. setting for different power levels for a fixed station at three different temperatures (say +10, +20 and +30 C) or five for a mobile station (with additional values for, say -50 and +70 C) should be enough. A few kilobytes should be more than enough to get the sample points for interpolation. As for me and most other amateurs, I'm quite happy with an S-meter reading which puts signal strength reports in the right ball-park. Very strong, very weak, or indifferent. Exchanging RS(T) reports is so stupid, so I try to avoid it. The only meaningful exchange would be to exchange the field strength (V/m) or the power density (W/mē) in order to study the propagation conditions. Even if we could reliably measure the antenna terminal voltage (e.g. in dBuV) or power (dBm), we still would have to measure the antenna efficiency and capture area to get any meaningful readings. Paul OH3LWR |
Best Way to build a calibrated S meter
"Reg Edwards" wrote in message ... wrote Only by coincidence and only in a 50 ohm system elsewhere S9 is 50 uV (microVolts) equals 50pW. ========================================== Allison, I'm afraid you are not quite correct about coincidences. The beauty of calibrating an S-meter in watts is that the result is independent of receiver impedance. Of course, there must be a conjugate match between antenna and receiver. And there usually is. This is taken care of in the calibration process. The received signal STRENGTH is indicated in watts, which is all one wants to know. When reading the meter, who cares about what impedance the measuring instrument happens to be? To sum up : There is a transmitter of given power output. There is a radio path which is an attenuator, And there is a received signal signal strength meter which indicates watts. The overall loss between transmitter and power meter may be deduced in terms of decibels. Professional radio engineers do it all the time. It can be misleading to think in terms of S9 = 50 uV when one doesn't know what the receiver input impedance is. ---- Reg, G4FGQ Tradionally S-9 has been defined as 50uV, but there really is not a standard. The S numbers are taken from the old R-S-T system of reporting signals (Readability, Strength, Tone). While the meter can be calibrated using a calibrated signal generator the reading for 50uV will vary with the gain of the receiver so it will not be the same on all bands or even from one end of a band to the other. Some receivers have meters calibrated in something other than S units. For instance, the old Hammarlund SP-600. The meters on these are usually calibrated for RF in db above 1 uv. But, the same thing applies, the receiver gain varies with band and frequency, the AGC is not linear, and the reading only an indication of relative strength rather than any actual value. -- --- Richard Knoppow Los Angeles, CA, USA |
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